As a method of printing arbitrary characters on a medium serving as a printing target such as a paper, a passbook and the like, an impact dot printer of a wire dot system is widely known. In such an impact dot printer, the medium serving as the printing target is arranged between a print head and a platen. An ink ribbon impregnated with ink is arranged between the print head and the printing medium, and when printing, the ink impregnated in the ink ribbon is sequentially transferred to the medium. Since the ink is sequentially transferred to the medium, if a large number of characters are printed, the amount of the impregnated ink is decreased and thus the thickness of the printed characters becomes thinner and thinner. The user confirms the printed characters through observation so that the ink ribbon is exchanged when the user feels that the printing concentration becomes thin. However, in some cases, the ink ribbon for exchange is not always possessed. Thus, in order to prevent the printing from being carried out continuously just in a low printing concentration before a new ink ribbon is obtained, it is also known that the ink in the ink ribbon is detected and the impact strength of the dot printer is changed according to the detection result. Though it is possible to improve the printing concentration when the impact strength is strengthened to prevent that the printing concentration becomes thin as the ink in the ink ribbon is reduced, there is still a problem that the noise may be generated due to increase of the impact strength, or the pin of the dot printer head may be broken.